The present invention relates to fluid coupling devices of the type including valve means operable to control the quantity of fluid in the fluid operating chamber. More particularly, the present invention relates to fluid coupling devices of the type including a valve element which is actuated to move in an axial direction between the open and closed positions.
Remotely actuated fluid coupling devices are especially adapted to the use of valve elements which move in an axial direction, because, in a remotely actuated coupling, movement of the valve is usually achieved by means of a device such as a solenoid or fluid pressure piston arrangement which inherently has an axial output movement. It should be understood, however, that the present invention is not limited to use in a fluid coupling device which is controlled by a remote sensing device. The present invention could also be used advantageously in a fluid coupling device in which the valve element is actuated in the manner shown in U.S. Pat. No. 3,144,922.
Axially movable valve elements in fluid coupling devices may have certain advantages over rotary valve elements, especially with regard to the simplicity of the valve actuation mechanism. However, there have been several major problems associated with axially movable valve elements. A first problem is the amount of force required to move the valve element axially and uncover the fill port. One cause for the high valve actuation forces is a phenomenon known as "stiction" wherein a flat portion of the valve element is in face-to-face engagement with the adjacent port plate with a thin film of viscous fluid therebetween, such that separation of the valve arm from the port plate requires a certain amount of force to break the fluid film.
Another cause of the high valve actuating force is the fluid pressure head acting to hold the valve element against the port plate. This pressure head is caused by the centrifugal force acting on the fluid as the coupling rotates, and the actual force on the valve arm is equal to at least the product of the fluid pressure against the arm and the area of the fill port being covered by the valve arm. The effect of the fluid pressure head on the valve arm, both for the prior art device and for the present invention, will be described in greater detail subsequently in the specification. High valve actuation forces are especially undesirable in the case of an electricallyactuated fluid coupling in which the size and weight of the solenoid increases exponentially in order to achieve an increase in the output force of the solenoid.
Another problem which has been observed in regard to axially movable valve elements is the lack of an accurate, predictable correlation between the amount of valve lift (axial movement) and flow through the fill port. As is understood by those skilled in the art, when a flat valve arm is lifted from a port plate, the effective port area is not merely equal to the area of the port, nor is the effective port area equal to the product of the circumference of the port and the amount of lift. Instead, it has been found that because of such factors as the flow characteristics of the fluid, fluid viscosity, speed of the coupling, etc., actual valve lift must be substantially greater than the theoretical valve lift in order for the effective port area to be equal to the actual area of the port. This situation makes it even more difficult than usual to achieve a desired relationship between the external condition, such as ambient air temperature or coolant temperature, and output speed (fan speed) of the coupling.